1. Field of the Invention
The present invention relates to a fluid mixer in which a first fluid flowing with subsonic speed through a Venturi tube is mixed with a second fluid fed via admixing openings, wherein in the Venturi tube, a first convergent section is provided which extends from an inlet cross-section up to the narrowest cross-section of the Venturi tube, and a second divergent section is provided which extends from the narrowest cross-section up to the outlet cross-section.
2. The Prior Art
Fluid mixers are used to mix two fluids, such as, e.g., air and fuel gas for a gas engine, in a desired ratio and as homogenous as possible. Such fluid mixers are usually constructed as Venturi tubes and operate in the subsonic range at substantially constant ambient pressure, Here, a first fluid, e.g., air, flows through the Venturi tube, wherein the speed of the fluid never reaches sonic speed. At the narrowest point of the Venturi tube, thus at the point at which the dynamic pressure (velocity pressure) is at a maximum and the static pressure (resting pressure) is at a minimum, a second gas, such as, e.g., gas or liquid fuel, is fed via openings in the Venturi tube. However, sonic speed is not reached in the narrowest cross-section which, as is well known, results in deceleration of the flow in the subsequent divergent part and thus in an increase of the static pressure. Due to the decelerated flow, the divergent nozzle region is particularly sensitive with regard to flow separation of the fluid mixture flowing therethrough. Accordingly, a problem in existing fluid mixers is the flow separation in the divergent part of the fluid mixer and accompanying disadvantageous pressure losses.
De Laval nozzles are principally to be distinguished from such fluid mixers operating according to the Venturi principle. A de Laval nozzle is a nozzle for accelerating a compressible gas flow from a subsonic state to a supersonic state. For accelerating a gas in the subsonic region, a narrowing (convergent) contour is required. The incoming subsonic flow is accelerated in the convergent nozzle part up to narrowest critical cross-section to sonic speed and from there is further accelerated to supersonic speed. This is based on the physical fact that a supersonic flow is accelerated in a diffusor (in contrast to a subsonic flow which is decelerated in a diffusor). Thus, there is an accelerated flow in the entire de Laval nozzle and the static pressure decreases monotonically with the increasing speed. Due to the existing stability of the accelerated flow, flow separations of the gas flowing through are not important. The much more important effect in the case of supersonic flows in a de Laval nozzle is the so-called compression shock which is generated by sudden deceleration of the flow to subsonic conditions and involves significant losses. Whether such a compression shock occurs in a de Laval nozzle depends entirely on the pressure ratio between nozzle inlet and nozzle outlet and the ratio of minimum cross-section to outlet cross-section. Such de Laval nozzles are frequently used as rocket nozzles, wherein here also so-called double bell geometries are known which, on the one hand, shall prevent compaction shocks from occurring and, on the other, shall also form defined separation edges at which the flow shall separate at defined conditions so that for all altitudes, a de Laval nozzle as optimal as possible is available. Thus, a flow separation is induced here in a targeted manner. Such de Laval nozzles are known, e.g., from EP 862 688 B1, WO 00/34641 A1, or U.S. Pat. No. 3,394,549 A.
It is therefore an object of the present invention to provide a fluid mixer which operates according to the Venturi principle and is insensitive to flow separations in the divergent part.